DESCRIPTION: About 16 million primary endodontic treatments are performed annually in the U.S., to treat pulpal and periradicular pathoses. Pulp necrosis and the development of periradicular lesion results in significant reduction in success of endodontic therapy, a significant percentage of tooth loss and may be associated with significant infections causing pain and swelling. The microorganisms associated with the pathogenesis, the increased severity of clinical symptoms, and the resistance to healing following treatment of endodontic infections has not been adequately characterized. We hypothesize that symptomatic endodontic infections and the absence of healing following adequate endodontic therapy are associated with the presence in the root canal of specific virulent microorganisms. We further hypothesize that microbial survival in the root canal system following endodontic instrumentation techniques, leads to a reduction in long-term healing of periradicular lesions. The Aims of this research project are: 1) To identify the predominant cultivable and not-yet-cultivated bacteria and fungi in endodontic infections, 2) To test the hypothesis that specific bacterial and fungal species are associated with the development of symptoms or the resistance to treatment, and 3) To test the hypothesis that specific bacterial and fungal species that resist conventional endodontic instrumentation are predictive of long-term outcome of treatment. The long-term objectives of this research effort are to understand the microbial factors that could predict the development of symptoms and/or the failure of endodontic treatment, to develop more effective endodontic antimicrobial treatment modalities, that would lead to reduction in perioperative symptoms and increase in long-term tooth survival, and finally to allow a better understanding of the prognosis of endodontic treatment for practitioners and patients.